Programmed cell death in the nervous system is a naturally occurring process in mammalian development; however, abnormal apoptosis is the basis for many neuropathologies, e.g. Alzheimer's and Parkinson's disease and ischemic injury. The delicate balance between neuronal survival and death is regulated, in part, by the neurotrophins, which promote survival through binding to the Trks, a family of tyrosine kinase receptors, and induce apoptosis through a 75kDa receptor, p75. While significant progress has been made in elucidating the mechanisms by which the Trks promote survival, much less is known about how p75 induces cell death. Nerve Growth Factor binding to p75 activates a bifurcating signal, promoting both cell survival through the transcription factor NF-kB, and cell death through the stress-activated kinase, JNK. Thus the ultimate fate of a cell in the developing or lesioned nervous system can only be understood by elucidating how these signals are generated and regulated. To meet this goal, we identified a novel zinc finger protein, NRIF, which binds to the intracellular domain (lCD) of p75. Our findings suggested that NRIF plays a role in neurotrophin mediated apoptosis in vivo. Recently, we found that NRIF binds to a member of the cytokine receptor associated factor family, TRAF6, which can also bind to the lCD of p75, and that this interaction directed NRIF translocation to a subnuclear domain. Therefore, we hypothesize that both NRIF and TRAF6 are part of the p75-mediated apoptotic signaling pathway and that their interaction and nuclear translocation are required for this signal. To address this hypothesis, we propose three integrated specific aims: I) Determine the ability of p75 to induce apoptosis in the absence of NRIF and TRAF6 binding to the receptor. This will include analysis of the nrzf-I- and traf6-/- mice. II) Establish the functional significance of TRAF6-NRIF interaction. By mutating the sequences on each protein required for their association and nuclear translocation, we will assess how the interaction of these signaling proteins affects their ability to regulate NF-kB, JNK, and apoptosis. III) Determine the mechanisms by which NRIF regulates cellular viability. We will investigate whether NRIF promotes apoptosis through repression of transcription. These studies will reveal the molecular mechanisms by which neurotrophins regulate cellular viability in the nervous system. In particular, they will elucidate how the apoptotic response is generated through the p75 receptor, which has potential clinical applications in developing therapeutic strategies for a variety of neuropathologies.